Contamination of water and soils by heavy metals is a serious environmental concern. As one example, soil contamination by mercury and other heavy metals is a common result of mining operations. Such heavy metals typically leach into water that comes into contact with the contaminated soil. The metals therefore give rise to environmental hazards in respect of both the contaminated soil and the water contaminated thereby, which may find its way into drinking water supplies.
Various processes have been proposed for reducing the metal content of wastewater. It is known that flocculating agents are of use in such processes. For example, U.S. Pat. No. 5,720,886 (Iwinski) discloses a process for removing metals from mine wastewaters that uses an anionic polymer and a flocculent.
It is also known that a variety of compounds form complexes with metal ions, providing the potential to remove the metal ions from the compositions they contaminate. Lignin is commonly considered to be the precursor of humic and fulvic acids, which are major organic constituents of soils. The capacity of humic and fulvic acids to complex metal ions is well established. For example, U.S. Pat. No. 6,143,692 (Sanjay et al.) discloses a process for removing metals from water using humic acid.
Kraft lignin and lignosulfonates are two classes of lignin derivatives available commercially. They are produced as by-products of the sulfate and sulfite pulping processes respectively. Lignosulfonates have been used in processes for the removal of various organic contaminants from water. Such processes are disclosed, for example, in U.S. Pat. No. 5,736,032 (Cox et al.), U.S. Pat. No. 4,933,087 (Markham, Jr. et al.) and U.S. Pat. No. 5,308,499 (Dixon et al.).
Lignosulfonates and kraft lignin contain an abundance of oxygen-containing functional groups, which are capable of forming lignin-metal complexes with high stability through ionic and covalent chemical bonding. It would be desirable to be able to use these plentiful and inexpensive lignin derivatives in processes for the effective removal of heavy-metal contaminants from water, and for the stabilization of heavy metal contaminants in soils, so that their leachability from such soils is substantially reduced.